1. Field of the Invention
This invention relates to devices used for frost sensing in a "demand-defrost" refrigeration system. The device can be applied broadly to heatabsorbing surfaces and to measure surface condensation from the atmosphere. Typical heatabsorbing surfaces to which the sensor may be applied include an evaporator tube, fin or shell in a refrigerator or heat pump. Another typical surface on which surface condensation and accretion detection are important is an aircraft wing.
2. Description of the Prior Art
Previous frost sensors include capacitive, optical, and acoustical structures. Capacitive structures experience an increase in capacitance due to frost deposition between plates monitored. Optical modules experience attenuation or scattering of light due to frost deposition. Acoustical cavities or waveguides experience an attenuation of sound due to frost deposition.
All three frost detection methods have drawbacks. There is a lack of discrimination between frost and ice in capacitive or optical schemes. The lack of thermal contact to the evaporator may limit the accuracy of optical devices. The size of acoustic waveguides limits their direct placement within available spaces in an evaporator.